Police from a heavily criticised sex crime unit pressured a woman to drop a rape claim against a man who went on to murder his two children, a watchdog has said.

Jean Say killed his son and daughter two years ago when they went to stay with him for a weekend.

The earlier rape allegation against him was dismissed by a detective sergeant based in the Sapphire unit at Southwark, south London, who said the circumstances did not constitute rape because the woman "consented".

A report by the Independent Police Complaints Commission (IPCC) said: "There is no doubt from the evidence that the woman made an allegation of rape at Walworth police station which should have been believed and thoroughly investigated."

Deputy chairwoman of the IPCC Deborah Glass said: "There's no doubt this was an incredibly serious, shocking incident. We know with all the cases that we've dealt with that the consequences of not dealing with allegations of rape can be extremely serious. This is yet another tragic illustration of that."

The case sparked a wider investigation into the work of the unit between July 2008 and September 2009, the results of which were published on Tuesday. It is the fifth into the Southwark Sapphire squad and the ninth into Scotland Yard's handling of sex crimes.

The report found that the unit was "underperforming and overstretched" during the period in question. Victims were pressured into giving retraction statements, which meant that the alleged crime had not taken place and boosted detection rates.

The report said victims were closely questioned by a detective constable before talking to a specialist officer. This meant they were questioned repeatedly and went against standard practice that a victim should be believed in the first instance until evidence showed otherwise.

Scotland Yard said it welcomed the findings of the report. The force released a statement to say: "We have for some time acknowledged that previous investigation of rape and serious sexual assault in the MPS was below standard.

"The activities identified in this report came during that era and highlight specific issues within Southwark which resulted in unacceptable actions by local officers. It is as a result of such failings that we have made substantial changes to the investigation of rape and serious sexual assault, both in terms of structure and revised working practices."